Skill Level: Beginner
Mono Audio Amplifier Quickstart Guide
by MikeGrusin | January 03, 2013 |
This tiny audio amplifier is based on the Texas Instruments TPA2005D1. It can drive
an 8-Ohm speaker at up to 1.4 Watts; it won't shake a stadium, but it will provide
plenty of volume for your audio projects.
Quickstart
• Connect your line-level audio input to the IN + and - header.
• Connect your speaker to the OUT + and - header.
• Connect 2.5V to 5.5V to the PWR + and - header. (Don't connect anything to
the S (shutdown) pin yet.)
Send some audio to the input, and you should hear it on the speaker! If the sound is
not loud enough, if you hear buzzing, or if you'd like to add a volume control, read on.
How it works
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Traditional audio amplifiers use power transistors to multiply an analog input by a
certain gain to produce an analog output. This produces great sound, but the
transistors must operate in the intermediate region between on and off. Transistors
operate most efficiently when they're fully on or off, and waste a lot of energy as heat
when they're run in between.
The class-D amplifier is radically different. At any moment its output transistors are
either fully on or fully off, which is very efficient. But this means that the output isn't
true audio, it's a digital PWM-like waveform which is mostly on (VCC) for high output
values, and mostly off (GND) for low output values.
This PWM signal is clocked at 250kHz, which is much higher than the audio
frequencies being encoded (< 20kHz). Because speakers are mechanical devices
that can only respond to audio frequencies, the PWM signal is effectively filtered back
into the original audio frequencies by the speaker coil itself. All you hear is the music!
Check out the TPA2005D1 datasheet if you'd like to know more about this amplifier.
To use the amplifier
Connect your input source to the IN + and - header. Because the inputs are
differential, it technically doesn't matter which direction you connect them, but if one
of your inputs is ground or shield, connect that line to the - terminal. (See the Tips
below if you're having problems with hum).
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Connect your speaker to the OUT + and - header. If you're using one speaker the
polarity doesn't matter, but if you'll be using multiple speakers, ensure that you wire
them all the same way to avoid phase problems. The speaker can be 4 or 8 Ohms.
The maximum output of the amplifier is 1.4 Watts into 8 Ohms. You can drive smaller-
wattage speakers with this amplifier, but you should be careful not to turn up the
volume too high to avoid damaging the speaker. You can also drive speakers larger
than 1.4W at reduced volume. In general, larger speakers will sound much better than
smaller speakers; we've had great luck with old automotive speakers.
Connect a power source to the + and - pins. The power source can be from 2.5V to
5.5V, and should be able to source at least 280 milliamps if you want maximum
volume. The red LED will illuminate when the board is powered up.
Apply an audio signal to the input, and you should be able to hear it on the speaker.
To change the volume, either change the volume of the signal at its source, or add a
volume-control potentiometer to the board (instructions below).
Adding a volume control knob
If you wish, you can easily add a 10K volume control potentiometer to this board. The
volume control will let you control the volume by reducing the input signal from 100%
to 0%. Note that this will not make the amplifier any louder - if you'd like to do that,
see changing the gain resistors below. Here's how to add a volume control
potentiometer:
Remove the solder from jumper SJ1. This is the small blob of solder within the white
brackets on the bottom right of the board. The easiest way to do this is to apply solder
wick to the jumper, and heat it with your soldering iron. When you're done, ensure
that the two sides are electrically separated from each other.
Now, connect your 10K potentiometer to the three pads on the lower right of the
board. The bottom of the board has silkscreen showing the proper connections. You
can solder a trimpot with 0.1"-spacing on the pins (such as COM-09806) directly to
the board, or you can use wires to connect the board to a larger potentiometer in a
chassis, etc.
Note: Potentiometers come in two styles: "linear taper" and "audio (or logarithmic)
taper". Audio taper potentiometers have a logarithmic bias that give a more natural
feel in audio volume control applications like this one, but linear taper will also work
just fine.
How to use the shutdown pin
The SDN* input can be used to turn off the amplifier to save power when it's not being
used. When SDN* is disconnected, or connected to a high logic signal (> 2V), the
amplifier will function normally. When the SDN* pin is connected to ground or a low
logic signal (< 0.8V), the amplifier and LED will turn off. You can use this feature to
save power in battery-operated projects.
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Changing the gain resistors
The amplifier chip uses two fixed resistors to set the gain, which is how much the
input signal will be amplified. On this board, we're using 150K resistors as
recommended by the datasheet, for a gain of 2. If you would like the output to be
louder, you can install smaller resistors.
The gain equation for this amplifier is 2 * (150K / R). So if you use 100K resistors the
gain would be 3, for 50K resistors the gain would be 6, etc. The datasheet states that
the smallest value you should use is 15K for a gain of 20, but we've gone down to 3K
(gain = 100) with fair results.
The amplifier board has two positions to add your own through-hole resistors. These
are within the white rectangles on the top of the board. You do not need to remove
the existing surface-mount resistors, just put your through-hole resistors over the top
of them. (Leaving the SMD resistors in parallel with the new resistors will slightly
reduce the total resistor value, but this is generally not a problem).
For best results, the two gain resistors should be as closely matched as possible. If
you have a bag of identical resistors, you might measure them all with a multimeter
and pick the two that have the closest resistance.
Tips
The amplifier's output is only designed to be connected to something with a coil (a
speaker or magnetic transducer). Since the output is not a true analog signal, you
shouldn't expect to use this board as a preamplifier, etc.
The differential inputs of this board are safe to connect directly to floating-ground
audio signals such as from the MP3 Shield and MP3 Trigger.
If your audio source and amplifier have different AC power supplies (such as audio
coming from a desktop computer), you may hear a loud hum in the output. To fix this,
connect a jumper wire between the - side of the audio input and the power supply
ground (PWR - header).
Because the amplifier outputs a 250Khz PWM-like signal, it could potentially radiate
interference to nearby sensitive circuitry. For this reason, keep the wires between the
amplifier and speaker as short as possible.
Questions?
If you have any questions or problems, let us know at techsupport@sparkfun.com
and we'll do our best to help you out. We'd also like to hear about the cool projects
you're building with our parts!
Have fun!
- Your friends at SparkFun.
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